Stents and other stented implantable medical devices have been used for many years to treat a number of vascular medical conditions. Stents in common use are either of the self-expandable type, made for example of a shape memory material such as Nitinol, or separately expandable, such as by balloon expansion.
Stents and other stented structures are designed to fit within the vasculature of the patient and need to be appropriate for the size and shape of the lumen. For this purpose, they must be conformable and must not be of a nature that they can apply against the vessel walls forces which could damage or adversely affect the functionality of the vessel or of other organs nearby. In the case of self-expanding stents, for example, these will tend to return towards their memory shape and thus will impart a force on any regions of the vessel wall which deform the stent away from its memorised shape. Such force is beneficial in some instances. For example, the radial expansion force generated when the stent is radially compressed assists in holding the stent in position in the lumen and, in the case of a stent graft, ensures in maintaining a proper seal between the implantable medical device and the lumen wall. However, in other instances, this force is a hindrance. For cases in which the medical device is located in a curved lumen, for instance, the force generated by the device can cause stress on the lumen by urging this into an unnatural straightened configuration or by reducing the flexure of the lumen during movement of the patient or of the patient's organs.
It has been known to provide stents with variable flexure characteristics in order to improve the longitudinal deflection properties of the stent, for example by having a part of the stent which is more flexible. In an example, a middle section of the stent could be made less rigid in order to give the stent improved longitudinal flexibility.
There are numerous known methods for increasing the flexibility of parts a stented structure. One method involves making the struts and/or other parts thinner than other parts of the structure. Another method enables making these parts of a softer material. It is similarly possible to heat treat a part of the shape memory structure in order to raise its transition temperature, for example to significantly above body temperature. So doing causes that part to remain in its martensitic phase when in the patient and to remain malleable, so as to deform in a quasi-elastic manner, with no or little tendency to return to its original shape.
There is a problem, however, with increasing the flexibility of a part of a stent in a manner as described above. A more flexible, or weaker, part of a stent structure will lose radial and/or longitudinal strength, in dependence upon the component or components which are made more flexible. In the case where only the tie bars or other components which are intended to provide longitudinal flexibility to the structure are made more flexible, although the structure will be able to conform better to the shape of the patient's lumen, these will not provide longitudinal strength to that stent. This can cause the stent to fail to provide the necessary support to the lumen and possibly to fail to retain its position in the lumen, with possible risk of migration of the medical device.
Prior art stent structures are disclosed in U.S. Pat. No. 6,652,576, U.S. Pat. No. 6,997,947, U.S. Pat. No. 5,992,020, U.S. Pat. No. 6,527,799, U.S. Pat. No. 6,998,060 and EP-1,859,825.